The present invention relates to a heat recovery boiler and a hot banking releasing method thereof, and more particularly, to a heat recovery boiler for preventing hammering generated at a time of blowing a drain water of a superheater from a system such as power generation plant including the heat recovery boiler and a hot banking releasing method thereof.
In a recent thermal power generation plant, a conventional (steam turbine only) power generation plant has been replaced by a combined cycle power generation plant in the trend of high efficiency operation, diversified operations and shortening of starting time. This combined cycle power generation plant is a combination of Brayton cycle of gas turbine plant and Rankine cycle of steam turbine plant and mainly comprises an air compressor, a gas turbine, a heat recovery boiler and a steam turbine.
In these principal components, the waste heat recovery boiler corresponds to a heat recovery steam generator in a conventional power generation plant and uses an exhaust gas (waste heat) from the gas turbine plant as a heat source and feed water from the steam turbine plant as a source to be heated. The heat recovery boiler is designed so as to generate steam by heating the source to be heated and supply the generated steam to the steam turbine plant as a driving source.
In the combined cycle power generation plant, starting operation and stopping operation are repeated frequently by virtue of the convenience of quick start and stop of the gas turbine plant, which is known as Daily Start-Stop (DSS) operation or Weekly Start-Stop (WSS) operation. Furthermore, in the combined cycle power generation plant, for effective utilization of excess heat of steam at the time of stopping the operation of the power generation plant, an outlet damper is closed, and preheated drain water or feed water is enclosed in the steam drum, an evaporator or a heat exchanger, which is known as hot banking. In such hot banking, while the gas turbine plant and the steam turbine plant are shut down, the drum water is maintained at a high temperature in a vessel, and therefore, when the operation is re-started, the steam is supplied into the steam turbine plant without delay to the quick start of the gas turbine plant, which assists and contributes to the quick starting.
FIG. 9 is a schematic block diagram showing an example of a conventional heat recovery boiler (heat recovery steam generator) through the hot banking.
In this waste heat recovery boiler, a casing 1 having a long structure (generally rectangular in section) has an inlet side connected to a gas turbine 3 through a duct 2 and an outlet side connected to a stack 6 through a damper 5. The gas turbine 3 and the steam turbine 4 are coupled through a shaft.
In the heat recovery boiler, the long casing 1 incorporates numerous heat exchangers such as a superheater 7, an evaporator 9 with a steam drum 8, and an economizer 10 installed along the flow of exhaust gas EG (waste heat) supplied from the gas turbine 3.
The superheater 7 is disposed so as to cross the flow of the exhaust gas EG and provided with a connection (lead) pipe 11 for guiding a saturated steam from a steam drum 8 at the steam inlet (entrance) side (IN) and with a superheated steam valve 12 and a superheated steam pipe 14 to be connected to the steam turbine 4 through a steam control (governing) valve 13 at the steam outlet (exit) side (EX). Furthermore, in a bottom portion of a connection pipe line 7c of the bottom of the superheater, a drain pipe 17 to be connected to a blow-down tank 16 is provided with a drain valve 15.
The economizer 10 is accommodated in the long casing 1 and positioned at the downstream side of the exhaust gas of the evaporator 9. The economizer 10 is provided with a feed water pump 18 and a feed water stop valve 19 at an inlet side thereof and an outlet side thereof is connected to the steam drum 8 through a feed water flow control valve 20.
In the heat recovery boiler having such constitution, in a loaded operation, the drum water in the steam drum 8 exchanges heat with the exhaust gas EG during spontaneous circulation in the evaporator 9, and a gas-liquid two-phase flow is formed, and only the steam relatively light in specific gravity of this gas-liquid two-phase flow is supplied to the superheater 7 through a lead or connection pipe 11 and becomes a superheated steam, which is supplied to the steam turbine 4 through a superheated steam valve 12, the superheated steam pipe 14 and the steam control valve 13.
At a time of stopping the operation, the heat recovery boiler closes the damper 5 and also closes the superheated steam valve 12, the drain valve 15 and the feed waster stop valve 19, and hot water is enclosed in the superheater 7, the steam drum 8 and the economizer 10 to establish the hot banking as standby to be ready for the restart of the operation.
The heat recovery boiler shown in FIG. 9 has several problems, including the hammering at the time of releasing the hot banking state or condition.
The heat recovery boiler has a vertical meandering pipe line as the superheater 7, and during the hot banking operation, as shown in FIG. 10, while the saturated steam generated from the steam drum 8 flows into the superheater 7 through the connection pipe 11, heat is released spontaneously and the temperature declines and then turns out as a drain water DW, which is often collected in the bottom of the superheater 7. At this time, the heat recovery boiler increases in the volume of water due to inductive force applied in addition to the natural convection of the saturated steam from the steam drum 8 in accordance with the decreasing in volume of the drain water DW.
If the hot banking is released and the superheated steam valve 12 is opened in the heat recovery boiler while the drain water DW is collected in the bottom of the superheater 7, a pressure difference is produced due to residual pressure of the steam drum 8 between the steam drum 8 and the superheater 7, and this pressure difference causes to press the drain water DW. A jet collision force may be applied to the meandering parts and elbows of the superheated steam pipe 14 and the piping system may be exposed to erosion, damage or other accident. Accordingly, in the waste heat recovery boiler, prior to releasing of the hot banking, as shown in FIG. 11, the drain valve 15 of the drain pipe 17 is opened and the drain water DW is then blown out into the external blow-down tank 16.
However, at a time of blowing the drain water DW out of the system, the drain water DW is discharged quickly from the inlet (IN) side of the superheater 7, but is slow in discharge from the outlet (EX) side because of the resistance of the meandering piping arrangement. At this time, bubbles BU are formed when the drain water DW at the inlet (IN) side flows into the drain pipe 17. The bubbles BU are grown when getting into the drain water DW stagnant at the outlet (EX) side of the superheater 7 as shown in FIG. 12, and then disappear as the saturated steam from the steam drum 8 contacts repeatedly to the drain water DW. At this time, due to the contact of saturated steam to the drain water DW and extinction of bubbles BU, the hammering (vibration due to collision of energy) takes place. When this hammering phenomenon is increased, it may give unexpected damages to the piping, piping supports, piping heat insulator and other piping systems.